1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor and a method for manufacturing the same, and more particularly, to an insulating film for separating a substrate from the active layer of the thin film transistor.
This kind of insulating film includes an underlying film formed on the whole surface of a substrate and a gate insulating film of a thin film transistor of a bottom gate type (typically, an inverted stagger type). The present invention relates to an underlying film or a gate insulating film suitable for preventing the deterioration of a thin film transistor.
A semiconductor device in accordance with the present invention includes not only such a device as a thin film transistor (TFT) or a MOS transistor but also an electro-optical device such as a display device and an image sensor which have a semiconductor circuit constituted by these insulating gate type transistors. Further, the semiconductor device in accordance with the present invention includes an electronic device provided with these display device and electro-optical device.
2. Description of the Related Art
In recent years, because of the increasing need for the upsizing and the falling costs of a liquid crystal display, the liquid crystal display has been expanding its market instead of a CRT, in particular, in the field of an OA product.
A pixel matrix circuit and a driver circuit can be made on the same substrate by making a thin film transistor (TFT) used in the liquid crystal display of polycrystalline silicon. Further, the fine patterning of the polycrystalline silicon has enabled a high aperture ratio and a high definition display.
In order to realize the further lower price of the liquid crystal display, it is required to use a glass substrate as a substrate. Accordingly, a research on a technology for manufacturing a TFT at a process temperature of from 600° C. to 700° C. or less has been carried out.
Since the glass substrate contains a lot of impurity ions such as Na+ or the like, it is necessary to form an underlying film made of silicon oxide, silicon nitride, or the like on the surface of the glass substrate to prevent the impurity ions from entering a semiconductor film.
When voltage is impressed by gate electrodes, an electric field is generated in an active layer, whereby the impurity ions in the substrate are attracted to the active layer. When the impurity ions enter the gate insulating film or the active layer through the underlying film or the gate insulating film, they vary electric characteristics, which results in incapability of guaranteeing reliability which does not vary with time.
In particular, in the case of a top gate type TFT, a region where a channel is formed is in contact with the underlying film and hence the quality of the underlying film has a profound effect on the characteristics of the TFT.
In a process for manufacturing a liquid crystal panel, a plasma CVD method is usually used for forming an underlying film or a gate insulating film. This is because the plasma CVD is performed at a low process temperature of from 300° C. to 400° C. and has a large throughput and can form a film in a large area.
Also, a silicon nitride (SiNx) or a silicon oxide (SiOx) is usually used as the underlying film. It is well known that the silicon nitride (SiNx) has the high effect of blocking impurity ions but has many trap levels, which present a problem in the characteristics of the TFT. The silicon oxide (SiOx) has advantages in that it has a wider band gap, better insulation, and lower trap level than the silicon nitride. However, it tends to absorb moisture and has the low effect of blocking the impurity ions.
Also, many films are laminated in the process for manufacturing a liquid crystal panel to manufacture a TFT, and the interaction of the internal stresses which is generated by the difference in the internal stresses between the films presents problems of varying the electric characteristics of the TFT such as a threshold, in some cases, warping the substrate, and separating the film.
The glass substrate is subjected to a heating treatment at a temperature close to its distortion point where amorphous silicon is crystallized and the glass substrate is shrunk by the heating treatment. Therefore, it is thought to be desirable that the internal stress generated in the insulating film formed on the glass substrate is a tensile stress to relieve the internal stress.